Recent advances in the production of digital signal processing devices, including single chip digital signal processors or application specific integrated circuits having DSP functionality, have allowed digital techniques to be exploited in mass-market communications environments. For example, first generation cellular mobile telephones are rapidly being superseded by second generation digital cellular mobile telephones, that provide at least three distinct advantages over the earlier analog systems. Firstly, they allow the overall size of the telephone to be reduced by reducing the requirement for large analog components. Secondly, they are less susceptible to signal degradation while at the same time transmitting signals that are virtually impossible to intercept by eavesdroppers. Thirdly, by the use of time division multiple access (TDMA), they allow greater use to be made of the available bandwidth, allowing a greater number of users to subscribe to mobile services while reducing operating costs for each individual circuit. Furthermore, TDMA technology substantially reduces the cost of base stations, given that a single radio transceiver may be used for establishing a plurality of calls.
Digital mobile telephones systems have been developed in the United States, in accordance with EIA IS-54 while throughout Europe the GSM standard has been adopted, allowing roaming techniques to be exploited across national boundaries. In accordance with the GSM system, a number of communications channels may be increased to a total of eight for each particular frequency pair. In addition, the system also employs frequency hopping such that, from one frame to the next, frames are transmitted on different frequencies.
It has been appreciated that, particularly in congested areas, customers will tolerate a degree of signal degradation while finding total service loss unacceptable to a greater extent. In order to exploit this situation, the GSM standard includes provisions for operating at half the normal transmission rate such that, rather than receiving a burst of data during each frame period, a burst in any one particular direction is only transmitted on alternate frame periods. Thus, using this so-called "half rate" mode of operation, the number of communication channels available for any particular transmission frequency is doubled. Similarly, further reductions in transmission rates may be envisaged, with data being transmitted on every fourth frame for example, although, at present, such a mode of transmission does not form part of the GSM recommendation.
Within the GSM recommendation, a plurality of techniques are employed in order to improve signal transmission while reducing the effects of noise and signal fading etc. The frequency hopping strategy has been identified above. In addition to this, an interleaving process is also performed, as described in the applicant's co-pending European patent application, published as 0 660 558.
Interleaving reduces the negative effects of burst errors but, as a result of the process, it is necessary for a plurality of transmitted frames to be received before data can be reassembled into its original order. Consequently, it is necessary for fast randomly accessible storage locations to be provided so that data may be buffered during the interleaving process and during the de-interleaving process. A mobile telephone will be required to buffer at least 1.5 blocks of source data (684 bits) in the full rate mode of operation, with 342 bits being required during half rate operation. Consequently, less buffering is required for the half rate mode of operation. However, during the coding and decoding process considerably more memory is required for the half rate mode of operation compared to that required for the full rate mode of operation.
It is known for DSPs to be provided with randomly accessible memory locations that are fast enough to supply instruction words to the DSP at the processor's normal operating rate. However, memory devices capable of operating at this rate are expensive and in any optimized design, the amount of this memory should be reduced in order to reduce overall costs. Often, a DSP will be required to perform a plurality of program types over a period of operation and under such circumstances it is often possible to replace a first set of memory instructions within DSP memory with a second set of DSP instructions within said memory. Such a procedure is commonly referred to as an "overlay" and as such may allow the amount of fast randomly accessible memory available to the DSP to be substantially reduced, for the same level of functionality.
However, a problem with known overlays is that a finite time is required for the new program instructions to be written to the fast executable memory. Although by its very nature, the fast randomly accessible memory may receive new instructions very quickly, the speed at which the transfer may take place will be determined by the speed at which the instructions may be read from bulk storage, possibly in the form of bulk storage associated directly with the DSP or, alternatively, from a larger area of slow storage associated with a slower microcontroller. In some situations, users may tolerate small interruptions in communication while a hand-over takes place from one type of operating system to another type of operating system. For example, in the United States, it is known for hand-overs of this type to take place when moving from congested city areas to more rural areas, given that digital systems tend to be used in the city areas while analog systems tend to be maintained within the rural areas. However, in accordance with the GSM recommendation, the maximum hand-over interval is only twenty milliseconds which, in most realizable mobile telephones would not provide sufficient time for the whole of the DSP's fast randomly accessible memory to be rewritten with a new program instruction set.